Industrially produced methionine mainly consists of DL-methionine, which is produced through chemical synthesis. When L-methionine is required, it is provided through production of N-acetyl-DL-methionine by acetylation of DL-methionine and subsequent enzymatic selective deacetylation of the N-acetylated L-methionine.
On the other hand, as for the production of L-methionine by fermentation, methods utilizing an L-methionine analogue-resistant mutant strain have been reported. However, their production amount is small, and factors affecting the L-methionine production have not been elucidated yet. Therefore, L-methionine is still one of the amino acids the most difficult to be produced by fermentation. For example, while methods utilizing Escherichia coli (E. coli) K-12 strain have been reported in Japanese Patent Laid-open (Kokai) No. 56-35992 and literature (Chattapadhyay, M. K. et al., Med. Sci. Res. 23, 775 (1995); Chattapadhyay, M. K. et al., Biotechnol. Lett. 17, 567-570 (1995)), any of these methods cannot provide L-methionine production amount sufficient for industrial use.
In E. coli, the biosynthetic pathway of L-methionine is partly shared with the biosynthetic pathway of L-threonine, and L-homoserine serves as a common intermediate. The first step of the peculiar pathway from L-homoserine to L-methionine is catalyzed by homoserine transsuccinylase (HTS). This enzyme has been known to suffer concerted inhibition by the final product, L-methionine, and a metabolite of L-methionine, S-adenosylmethionine (Lee, L.-W. et al., J. Biol. Chem., 241, 5479-5480 (1966)).
The nucleotide sequence of the metA gene encoding homoserine transsuccinylase of E. coli, has been reported by Duclos et al. (Duclos, B. et al., Nucleic Acids Res. 17, 2856 (1989)), and a method for obtaining a strain having a mutation for metA using resistance to an analog of L-methionine, α-methyl-DL-methionine (MM) has also been known (Chattopadhyay, M. K. et al., J. Gen. Microbiol., 137, 685-691 (1991)). It has been reported for Salmonella typhimurium that, the metA gene product, homoserine transsuccinylase, was an inhibition-desensitized type as for the inhibition by L-methionine and S-adenosylmethionine synthase (SAM) in an MM resistant strain (Lawrence, D. A. et al., J. Bacteriol., 109, 8-11 (1972)). However, the nucleotide sequence of the mutant meta gene has not been reported. Furthermore, it has been reported that a mutant having a sole mutation in metA did not secret L-methionine (Chattopadhyay, M. K. et al., J. Gen. Microbiol., 137, 685-691 (1991)).
It has also been revealed that the expression of the genes including metA of the enzymes for the reaction by homoserine transsuccinylase and subsequent reactions in the peculiar biosynthetic pathway of L-methionine suffers inhibition by a repressor which is a metJ gene product (Green, R. C. Biosynthesis of Methionine in “Escherichia coli and Salmonella Cellular and Molecular Biology/Second Edition”, ed. Neidhardt, F. D., ASM Press, pp. 542-560 (1996)). It has also been known that the metJ gene is adjacent to the metBL operon in a reverse direction, which operon consists of the metB gene coding for the second enzyme of the peculiar biosynthetic pathway for L-methionine, cystathionine γ-synthase, and metL coding for aspartokinase-homoserine dehydrogenase II (AK-HDII) (Duchange, N. et al., J. Biol. Chem., 258, 14868-14871 (1983)).
It has been suggested that metK coding for S-adenosylmethionine, which catalyzes the metabolic reaction from L-methionine to S-adenosylmethionine, should be an essential enzyme (Green, R. C. Biosynthesis of Methionine in “Escherichia coli and Salmonella Cellular and Molecular Biology/Second Edition”, ed. Neidhardt, F. D., ASM Press, pp. 542-560 (1996)). Furthermore, it has also been known that a metK mutant strain can be obtained based on resistance to a methionine analogue such as DL-norleucine and ethionine (Chattopadhyay, M. K. et al., J. Gen. Microbiol., 137, 685-691 (1991)), and can increase the expression of the enzymes of the peculiar biosynthetic pathway of L-methionine (Greene, R. C. et al., J. Bacteriol., 115, 57-67).
As mentioned above, there have been reported enzymes involved in the L-methionine biosynthesis and genes therefor to some extent. However, only few findings that directly lead to the production of L-methionine by fermentation have been obtained, and hence hardly applied to breeding of L-methionine-producing bacteria.